This invention relates to a shank structure or holder for holding an annular hole cutter. This structure is especially effective in case of using a simple drilling machine, such as a portable drilling machine, which cannot fix a relative position of a cutter and an object to be worked.
As disclosed in U.S. Pat. No. 3,609,056, for example, the annular hole cutter means a tool including a cylindrical cup-shaped body having saw-tooth-like cutting edges on its open end and a coaxial shank fixed to its closed end for cutting an annular groove and finally forming a large circular hole by pressing the cutting edges against the object and rotating the shank. In this case, there will be no trouble when a relative position of the cutter and the object is certainly fixed as in the case of large stationary drilling machine. However, it is very difficult to maintain the center of drilled hole when it is impossible to fix the relative position of the cutter and the object as in the case of portable drilling machine. In such case, therefore, it is a general practice, as shown in the above U.S. patent, to fix a drill coaxially with the shank of the annular cutter with its top a little projecting from the cutting edges of the cutter and to form a center hole first by the drill and then cut a large hole by the cutter using the center hole as a guide.
When a hole is formed by such annular hole cutter, a disc-like chip is naturally left within the "cup" of the cutter and it is a very troublesome and time-consuming work to remove this chip.
The annular hole cutter disclosed in U.S. Pat. No. 4,408,935 is adapted to automatically remove such chip. The shank structure of this cutter has a coaxial cylindrical cavity therein in which a coiled spring and a center pin are inserted. The center pin is constantly urged outwords by the spring and adapted to rotate with the cutter. While the initial appearance of this structure is similar to the cutter structure of the above U.S. Pat. No. 3,609,056, if the cutter is pressed against the object, the center pin is pushed into the cavity against the spring without drilling a hole and, when the drilling operation is finished, it is returned to the initial position by the spring to push out the chip left in the "cup". In this structure, however, it is very difficult to obtain the abovementioned "self-centering effect" since it cannot form a guide hole in the center.